The refractive-diffractive switchable lens or diffractive switchable lens of the present invention can be applied outside or within ophthalmic application. In latter case the lens can be called diffractive switchable ophthalmic lens. Ophthalmic lens in the present invention is defined as a diffractive switchable lens suitable for placement outside the eye such as spectacle lens or contact lenses or inside the eye such as aphakic and phakic intraocular lenses or implants placed in posterior or anterior eye chamber and also included are artificial corneas and corneal implants or inlay. For detailed explanation of the lens of the present invention, the ophthalmic application for presbyopia correction is used as a preferred embodiment.
A fixed single power lens provides good quality vision but only within a small range of viewing distances that is usually significantly narrower than the range required from near to far vision. The resulted vision deficiency is called Presbyopia. There is a significant effort to develop a lens for Presbyopia correction in a form of multifocal refractive or diffractive type lenses that provide multiple foci and also in a form of accommodating lenses that may change their optical surface shapes or positions inside the eye for ocular power increase for near vision. Diffractive switchable optical element according to the present invention is a lens that consequently changes the image positions between distance and near foci or fields by directing light to refractive focus and a different focus position corresponding to a diffraction order. Usually but not necessary the power corresponding to diffraction order is higher the refractive power and this difference is called Add power as a traditional reference to a power difference between near and far foci in presbyopia correction. A refractive-diffractive switchable ophthalmic lens implanted inside the eye is also called diffractive accommodating lens as switching between far and near foci occurs under the action of ciliary muscle. It is important to note that diffractive switchable lens according to the present invention has application outside ophthalmic one in so called engineering optic (microscopes, telescopes, photo-objectives, etc.).
Portney' U.S. application Ser. No. 13/247,840, “Switchable Diffractive Accommodating Lens” explained accommodation, ciliary muscle action during accommodation and prior art involving in Presbyopia correction. The same Application also disclosed Sensor Cell that functioned for (a) sensing a need to focus for far or near and/or (b) actuation of an implantable accommodating lens (phakic, aphakic, corneal inlay) to focus at far or near by a direct interaction with ciliary muscle at the location of the ciliary spur. Equivalent Sensor Cell can also be used with a diffractive accommodating lens of the present invention.
The advantage of the diffractive switchable optic in switching between far and near foci over a refractive optic was described in Portney' U.S. application Ser. No. 13/247,840, “Switchable Diffractive Accommodating Lens” and also by a large group of researches: Li G, Mathine D L, Valley P, et al. “Switchable electro-optic diffractive lens with high efficiency for ophthalmic application”, Proceedings of the National Academy of Science of the USA, 2006; 103: 6100-6104, in the application to the spectacle lens. The operation of the described by Li G, et al spectacle lens was based on electrical control of the refractive index of thin layer of pneumatic liquid crystal by creating volume diffractive element through refractive index modulation. Though the approach is feasible, it is complicated and expensive to execute and it also requires electric field for its operation which is a challenge for ocular implant or contact lens application. Haddock et al. in US Patent Application 20090256977 introduced further improvement to the above volume diffractive lens manufacturability. The spectacle lenses according to the above design were released by PixelOptics under EmPower name. Similar technology is under development for accommodating IOL by ELENZA, Inc.
A diffractive switchable lens according to the present invention incorporates a switchable cell and optical surfaces. A switchable cell of the present invention is an opto-mechanical device that utilizes mechanical optical switching between far and near foci by changing the optical surface shape between refractive continuous form for one focal position and diffractive form of a periodic structure for a different focal position. Refractive surface focusing ability depends on surface curvatures and diffraction surface focusing ability depends on a diffraction surface periodic structure. An optical surface can be part of a switchable cell or part of a separate optical element that together form diffractive switchable lens. Switchable cell is the center of the present invention for operation of a diffractive switchable lens.
Diffractive optic has been described by a huge volume of papers. For instance, D Fakies and D M Morris, “Spectral properties of multiorder diffractive lenses”, Apply Optics 1995; 34(14): 2462-2468, described diffractive optic of blazed shape, so called surface relief shape of different diffraction orders. This type of diffractive optic is called Kinoform that allows 100% of light to be directed to a single focal position formed by a single diffraction order or multiorder per Fakies and Morris explanation. Diffraction zones forming diffraction surface periodic structure can be of different profiles, rectangular, sine-wave for instance, as well as blazed surface of different profiles (parabolic, linear, sine) of a single order or multiorder composition and any one or a combination of them can be applied to a diffractive switchable lens of the present invention for so called diffraction guiding surface introduced below.
The advantage in using diffractive element is that the focal length of selected diffraction order is defined only by the periodic structure of the diffraction surface which can be built into the diffraction surface design. By the law of formation of a diffraction order, light can only be channeled along discrete diffraction orders of the diffractive lens where light constructive interference can take place thus providing predetermined separation between refractive and diffractive foci or Add power in ophthalmic application. Thus, a difference between refractive and diffractive focal positions of the diffractive accommodating lens of the present invention is predetermined by the switchable cell design itself and not by an external factor as in all refractive accommodating lens designs which makes them inherently unpredictable.
An image is physically formed at a given focus of a diffraction order if a significant percent of total light is actually channeled alone the given diffraction order. This depends on the light phase shift introduced by each diffraction zone, i.e. diffraction groove height or periodic surface modulation amplitude. There are a lame number of papers explanting a diffractive lens light distribution and V. Portney, “Light distribution in diffractive multifocal optics and its optimization”, J Cataract Refract Surg 2011; 37:2053-2059, offers an explanation in terms of multifocal optics and for blazed diffraction surfaces.
In a simple paraxial form the circular diffraction zones, also called grooves, echelettes or surface-relief profile, can be expressed by the formula:rj=√{square root over (2·j·m·λdesign·Fm)}  (1)where grooves radii (rj) create spherical wavefront of focal length (Fm) for a given diffraction order (m) and design wavelength (λdesign).
In case of surface relief profile and in paraxial approximation, the blaze material thickness (Hm) to produce 100% efficiency at the focus of m-order diffraction is
                              H          m                =                              m            ·                          λ              design                                            (                          n              -                              n                ′                                      )                                              (        2        )            where n=refractive index of the lens material and n′=refractive index of the medium adjacent to the diffraction surface.
In case of small spherical aberration, the periodic structure, i.e. diffraction groove periods of the diffractive optic is quite accurately defined by the Eq. 1 for a given focal distance. In case of a significant spherical aberration of a diffractive lens introduced to extend a range of foci around a focus of a selected diffraction order, the calculation of the groove shapes can be conducted numerically equivalent to the method described by Portney in the US Patent Appl. No: 20100066973 for multifocal diffractive lens.
Portney' U.S. application Ser. No. 13/247,840, “Switchable Diffractive Accommodating Lens” disclosed diffractive optic with specific adjustment of its periods to create positive spherical aberration which is the most effective for depth of focus increase at a diffraction order. The same form of the diffraction surface periodicity can also be applied to the guiding surface of the diffractive switchable lens of the present invention to expand a range of foci at its diffractive focus.